Weft feeding devices for weaving looms are apparatuses which are arranged between the loom and the thread reels which feed the weft to the loom, to perform the function of unwinding the thread off the reels and hence make it available to the weft insertion devices, keeping the thread tension within acceptable levels during the entire weft insertion operation, and hence avoiding the abrupt tension peaks in the thread which occur instead upon weft insertion in looms without weft feeders. This object is achieved through the presence, in the weft feeder, of a winding assembly which regularly and at a lower average speed takes the weft thread from the reels, accumulating it in successive coils on a stationary cylindrical drum whereon it hence forms a thread stock. Such stock is then collected, discontinuously and at high speed, by the weft insertion devices (launch nozzles or grippers) of the loom.
The weft feeder is an apparatus which has been in use in weaving looms for many years now, in particular since modern high-speed looms have been introduced, wherein the direct feeding from the reels has never been technically possible. During its evolution over the years, in addition to the basic functions recalled above, the weft feeder has acquired additional control functions which allow to verify the constant presence of thread in the critical points of the weft feeder, to adjust the amount of thread accumulated in the stock, to brake the outgoing thread to limit the dynamic effects determined by the abrupt acceleration during its collection by the weft insertion devices, to measure the length of the thread portion collected by the insertion devices, and hence to stop thread collection as soon as a predetermined length thereof has been supplied.
These different functions are obtained due to the presence, aboard the weft feeder, of a processing unit which operates on the basis of sophisticated algorithms, starting from electric signals for the detection of the thread presence/absence in correspondence of the above-said critical points of the apparatus. These electric signals are currently obtained, preferably with respect to the use of mechanical sensors, through pairs of emitting/receiving optic sensors arranged on the weft feeder so that the path of the optic radiation between a emitting sensor and a receiving sensor intercepts the thread path in a desired control position. Depending on the type of path of the optic radiation, and consequently of the positioning of the optic sensors on the weft feeder, current weft feeders divide into two categories.
In a first weft feeder category, both emitting sensors and receiving sensors are arranged on a support arm which projects from the main body of the weft feeder and extends parallel to the lateral surface of the drum, and the path of the optic radiation between each pair of sensors is obtained through a respective reflecting surface fastened to the lateral surface of the drum which faces said support arm, in a carefully preset position and angle.
In a second weft feeder category, the emitting sensors are instead arranged on the outer surface of the stationary drum which faces the support arm, while the receiving sensors remain in the position already described above on said support arm.